Recent emphasis on ecological concerns in the environment has spawned many efforts to solve the world's air pollution problems. Two major concerns worth noting are acid rain and photochemical smog. While many compounds contribute to theses problems, NO.sub.x plays an important role, imposing a significant threat to the environment and human health.
NO.sub.x is a family of compounds of nitrogen and oxygen, primarily, NO and NO.sub.2. NO.sub.x comes from a variety of sources, most notably cars, trucks and industrial plants. Specifically, NO.sub.x is produced by high temperature combustion systems, metal cleaning processes, and the production of fertilizers, explosives, nitric acid, and sulfuric acid. In many urban environments, automobiles and diesel engine trucks are the major sources of NO.sub.x.
NO is the stable oxide of nitrogen at combustion temperatures. Hence, it is more abundantly produced than NO.sub.2. However, at atmospheric conditions, the equilibrium between NO and NO.sub.2 favors NO.sub.2. Therefore, the effective control of NO.sub.x concerns both the control and removal of both NO and NO.sub.2 from exhaust gas streams.
Recently, methods of NO.sub.x reduction involving exposure of a gas stream containing NO.sub.x to HNCO have been disclosed. HNCO, also known as isocyanic acid, is an unstable, highly reactive, polymerizable gas at ordinary temperatures and pressures, and is a lachrymator, and thus is hard to handle and store. Free HNCO gas is unstable at temperatures ranging from about 25.degree. F. to about 450.degree. F., tending to polymerize within this range. Free HNCO liquid is unstable above 25.degree. F. (-4.degree. C.). Thus, free HNCO must be stored at temperatures and pressures outside its unstable ranges. As is known in the art, the precise temperature at which HNCO becomes stable in the liquid and gas phases is also dependent upon factors such as pressure and concentration.
HNCO also can be stabilized at ambient temperatures by bonding it to organic groups or molecules (forming an organic isocyanate), or by trimerizing it to form cyanuric acid. Cyanuric acid decomposes when heated, forming HNCO. The conversion of cyanuric acid to HNCO takes place at relatively high temperatures, such as from about 650.degree. to about 1400.degree. F.
There are disadvantages to the prior art methods for stabilizing HNCO. The high or low temperatures and low pressures required to store free HNCO require expensive and large equipment. The organic substitutents used to form organic isocyanates can generate extraneous and sometimes unacceptable compounds upon decomposition of the substituted HNCO or require excessive decomposition temperatures.
Adsorption (or, more broadly, reversible bonding) of an active material on a substrate is widely used in both liquid and gas phase treatments. Adsorption is the ability of a substance or substrate to hold or concentrate gases or liquids upon its surface. Liquid-phase applications include decolorizing, drying, purification, salt removal, and odor, taste and color removal. Gas-phase applications include separations, solvent recovery, dehydration and odor removal.